The aim of the proposed research is to refine our understanding of the role of birth order and fimily size for intellectual competence. Our previous analysis of the data for a total population of some 400,000 19-year-old Dutch males showed clearly that birth order and family size had independent effects on a number of psychometric measures; as birth order and family size increased, test scores decreased. Our present objective is to refine the nature of the relationships found by examining interrelationships among test performances and by examining additional variables which are pertinent to an interpretation of birth order and family size effect. These variables include education, health status, stature and sex of sibling. We also plan to study special subgroups within the data set: individuals from one-child families, siblings in 2-child families (for analyses of the relation of spacing to the effects noted) and the mentally retarded. BIBLIOGRAPHIC REFERENCES: Belmont, L., Stein, Z.A. and Susser, M.W. Comparison of associations of birth order with intelligence test score and height. Nature, 1975, 255: 54-56.